RON (Recepteur d'Origine Nantais) or alternatively STK (stem cell derived tyrosine kinase) is a novel receptor tyrosine kinase belonging to the Met proto-oncogene family. The only known ligand for RON is MSP (macrophage stimulating protein). MSP is constitutively expressed in liver and circulates as an inactive form requiring proteolytic conversion for activation. Activation of RON by MSP stimulates macrophage shape change, chemotactic migration, and phagocytosis, however, RON activation also results in blocking endotoxin or cytokine-induced expression of inducible nitric oxide synthase (iNOS) by murine peritoneal macrophages, suggesting a dual role in inflammation. Expression of iNOS is required for bactericidal and tumoricidal actions of macrophages. However, NO is also an important cause of tissue destruction in inflammation and mediates some of the adverse hemodynamic consequences of septic shock. Therefore, understanding the mechanisms of iNOS expression by macrophages is important and likely to be clinically useful. The hypotheses underlying this proposal are that: 1) RON expression is regulated such that not all populations of tissue-derived macrophages express RON; 2) RON activation inhibits iNOS expression at the transcription level; 3) Specific inhibition of iNOS is determined by the intracellular domains of RON that transduce a unique signaling pathway. To test these hypotheses, the expression and activation of RON in regulation of LPS or IFN-gamma-induced iNOS synthesis by different types of tissue macrophages will be determined. The effect of RON activation on IFN-gamma or LPS-induced signaling pathways essential for macrophage iNOS expression will be studied. And the potential tyrosine-containing domains in the RON C-terminal tail that transduce iNOS inhibitory signals will be identified. The applicant proposes that this work is important for several reasons. First, it will provide the basis for better understanding of iNOS regulation by endogenously produced inhibitors. Second it will facilitate understanding of the role of MSP in regulating macrophage activities in inflammation. Further, this work may lead to novel approaches for decreasing tissue destruction in inflammation.